Transient and Intermittent Crack Growth During Embrittlement of 7075-T651 Aluminum by Mercury

8/12/2019 Transient and Intermittent Crack Growth During Embrittlement of 7075-T651 Aluminum by Mercury

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S c r i p t a M E T A L L U R G I C A V o l . 2 3 , p p . 3 3 9 - 3 4 4 , 1 9 8 9 P e r g a m o n P r e s s p l cP r i n t e d i n t h e U . S . A . A l l r l g h t s r e s e r v e d

T R A N S I E N T A N D I N TE R M I T T E N T C R A C K G R O W T H D U R I N GE M B R I T T L E M E N T O F 7 0 75 T 6 51 A L U M I N U M B Y M E R C U R Y

Y. Liu and R . G . Hoag landDepa r tmen t o f M echan ica l & M a te r i a ls Eng inee ringWas h ing ton S ta t e Un ive rs i tyP u l lm a n , W A 9 9 1 6 4( R e c e i v e d D e c e m b e r 2 0, 1 98 8 )

Introduct ionThe in t e rac t ion o f a tom s o f l i qu id mercu ry w i th the a toms a t t he t ip o f a c rack in s o l id a luminum g rea t lyreduces the s t re s s in t ens i ty requ i red to p ropaga te tha t c rack , and l ike hydrogen embr i t t l emen t (HE) and s t re s sco r ros ion c rack ing (SC C ) in aqueous env i ronmen t s , t h i s example o f l i qu id me ta l embr i t t l emen t (LM E) i s gene ra l lyrega rded a s a type o f env i ron men ta l ly - indu ced f rac tu re (1 -5) . A l s o l ike the s e o the r fo rms o f embr i t t l emen t , t heway in w h ich th i s in t e rac t ion d imin i s he s c rack t ip de fo rm a t ion o r p romo te s c rack ex tens ion , o r bo th , is no t c l e a ra l th o u g h a n u m b e r o f p o s t u l a t e s h a v e b e e n a d v a n c e d . S o m e m e c h a n i s m s w h i c h h a v e b e e n s u g g e st e d , su c h a sthos e invo lv ing the weaken ing o f in t e ra tomic bonds a t t he s o l id s u r face a t t he c rack t ip due to the p re s ence o femb r i t t l i ng s pec ie s o r a s h i f t in b i a s e i the r toward o r aw ay f rom c rack - t ip d i s loca t ion nuc lea tion (5 -8 ) , may becommon to a l l t h ree types o f b r i t t l e f ra c tu re s . Wha teve r the mechan i s m(s ) , i t i s a l s o c l ea r tha t t he re a re o the rf a c to r s o p e r a ti n g . F o r e x a m p l e , m o s t o f t h e w o r k r e p o r t e d o n m e r c u r y e m b r i t tl e m e n t o f a l u m i n u m c o n c e r n s o n lyh igh s t reng th a l loys , a s low s t reng th , o r pu re a lum inum, i s no t s eve re ly embr i t t l ed . In add i t ion , c e r t a in s pec ie s int h e m e r c u r y m a y b e i m p o r t a n t , a n d , i n t h i s r e g a r d , W h e e l e r a n d H o a g l a n d ( 9 ) h a v e s u g g e s t e d t h a t o x y g e n d i s -s o lved in the mercu ry i s av a i l ab le to fo rm p ro tec t ive a luminum ox ide , a r e ac t ion tha t com pe te s w i th the embn t t l e -

men t . Th i s s ugges t ion was an a t t empt to exp la in the non-un ique v -K cu rves they obs e rved . Thus , s t ruc tu re o f thev -K cu rve can revea l s om e fea tu re s o f the c rack ing mechan i s ms .Th i s pape r p re s en t s s om e re s u l t s wh ich s how in te rm i t t en t and t r ans ien t c rack g rowth beh av io r o f a h ighs t reng th a lum inum a l loy em br i t t l ed by l iqu id mercu ry . R u n-a r re s t - run type o f c rack ing behav io r r e s u l t ed in nand m - s haped cu rves on the c rack ve loc i ty (da /d t ) v s . s tr e s s in t ens i ty (K) d iag ram. Whi le va r i a t ions in oxygenac t iv i ty in the c rack t ip env i ro nme n t (9 ) ma y p lay a ro l e in p roduc ing non -un ique v -K beh av io r , t he re s u l ts r e-po r t ed he re s ugges t t ha t fo rma t ion and rup tu re o f l i gam en t s du r ing c rack ex tens ion m ay a l s o be a t work in a waytha t c aus e s the s t re s s in t ens i ty a s s oc ia t ed wi th the f i e ld loca l to the c rack t ip (K t ip ) to va ry s toch as t i c a l ly a s thec rack ex tends .

Exoerim enta l Deta i lsDou b le can t i l eve r beam s pec imens us ed in c rack g rowth t e s ts we re mach ined ou t o f a 12 .5 m m th ick c l ad7075-T651 p la t e supp l i ed by Ka i s e r A luminum . The p la t e c ame w i th a ha rdnes s o f R ockw e l l B 91 .5 The geome-

t r ie s o f t h e D C B s p e c i m e n s a r e s h o w n i n F i g u r e 1 . S p e c i m e n s w e r e m a c h i n e d s u c h t h a t c r a c k p r o p a g at i o n oc -cu r red in the long t rans ve rs e o r i en ta t ion wi th the c rack p lane pe rpend icu la r to the ro l l ing d i rec t ion ( the L-Tor ien ta t ion ) . Spec im ens we re s ide -g roov ed to a ne t t h i cknes s o f 4 .6 mm .O n e d r o p o f c o n ce n t ra t e d H F w a s p l a c ed a t t h e m a c h i n e d s lo t i p t o r e m o v e t he o x i d e a n d t h e n a p p m x ~

m a t e l y 2 m l o f l iq u i d m e r c u r y w a s a p p l i e d t o t h e c r a c k ti p. f t e r 5 r ai n. h e s p e c i m e n s w e r e l o a d e d a t a c r o s s h ea ds p e e d o f 0 . 1 5 2 m m / s c c . E a c h s p e c i m e n w a s l o a d e d u nt il a s i gn i fi c an t e p a r t u r e f r o m l in ea ri ty c c u r r e d in t hel o a d - d i s p la c e m e n t c u rv e w a s n o t i c e d i n di c a ti n g o n s e t o f c r a c k e x t en s i on . A f t e r c r a c k e x t e n s i o n c o m m e n c e d t h el o a d i n g c o m m a n d w a s e i t he r s w i t c h e d t o f i x e d l o a d ( i n c r ea s i n g s t re ss i n te n si t y w i t h c r a c k g r o w t h ) o r t o f i x edc r o s s h e a d ( d e c r e a s i n g s t r e ss i n te n si t y) . s n o t e d b e l o w , c r a c k g r o w t h w o u l d o c c a s i o n a l l y st o p , m o m e n t a r i l y ,t h e n r e c o m m e n c e . E v e n t u a l l y , u n d e r f i x e d g r i p c o n d it i o n s, c r a c k g r o w t h w o u l d s t o p , p e r m a n e n t l y , a n d t h ea p p l i e d K a t t h a t a r re s t i s r e f e r r e d t o a s a t h r es h o ld . T h e s p e c i m e n c o u l d t h e n b e r e l o a d e d a n d a n o t h e r t e~ tc o n d u c t e d . G e n e r a l l y f i v e o r s i x r u n -a r r es t e v e n t s c o u l d b e c o n d u c t e d o n a s i n g l e s p e c i m e n . H o w e v e r , i f t h e n i n e

3 3 90 0 3 6 - 9 7 4 8 / 8 9 3 . 0 0 + . 0 0C o p y r i g h t ( c ) 1 9 8 9 P e r g a m o n P r e s s p l c


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3 4 0 E M B R I T T L E H E N T O F A L B Y H g V o l . 2 3 N o . 3

between tests was too long (> 20 min.), the mercury would crawl out of the crack and embrit tlement would nolonger occur.4 . 60 m m

50.8 mm ~

o~ 2 5 4 m m

12.7 m m

19.1 mm

FIG. 1. Double cantilever beam specimen used in the crack growthtests

Duri ng the tests, load anddisplacement were recordedsimultaneously at a rate of 5 Hz on a12bit digi tal recorder A five-pointweighted running average of therecorded data points was calculated toreduce high frequency noise. Thesmoothed data were used to calculatethe crack length and the applied stressintensity (K) using Kanninen'sequation for this geometry(10). Crackvelocity (da/dt) was calculated byfour-point, central differences Theaccuracy of crack length calculated bythis method is on the order of 1.0rnm, but the sensitivi ty in determiningda/dt is about +5.0 x 10 5 m/sec.

Resul~Examples of load time records in which the crosshead was stopped after the crack began to extend areshown in Figures 2 and 3. They are the results of two events of fLxed crosshead run-arrest from a single DCBspecimen. Shown in Fig. 4 is the corresponding da/dt-K curve. The crack velocity accelerates from a low level,reaches its maximum value and decreases as a resul t of decreased K and finally arrests at the K threshold Thecrack acceleration produced an inflexion on the load-time curves, and correspondingly, a n shaped transient riseon the v- K diagram. During the crack propagation, occasionally intermi ttent crack growth was detected asindicated by the flat step on the load-time curve and the valley-like m shaped structure on the v-K plot,indicating that, momentarily, crack growth stopped then re,commenced.

ot=0

11001000

90087

~ , ~ G R I PRXED

a600

I500 t . . . .150 200 250 300 350 400TIME Seconds)

FIG. 2. Plot of load vs. time from a run-arres t eventwith constant displacement control. Steps on thecurve represent intermittentcrack growth behavior.

0t=

0J

600500400300200

u - , i n I I l

- - - 3 G . ..ix

| i l l i I I I ~ I I

320 340 360 380 400 420 440 460TIME Seconds)

FIG. 3. Plot of load vs. time from a run-arrestevent with constant displacement control.Inflection on the curve represents a momentaryslowing of crack growth.

With constant-load control the stress intensity increases with crack extension. An example of resultsfrom a fixed load test is shown in Figure 5 in which a loop in the K-v curve was generated that signals thefailure of the machine to maintain a constant load due to the speed of the crack. Figure. 6 is the K-v diagram offive run-arrest event s which were obta inedfro m one DCB spegimen. The var iation suggests that the thresholdsare never the same but fall in a range general ly from 4 to 9 Mpa',/m. The maximum crack velocity detected in thesetests was 0.02 m/s.


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001

,0001 -4 5 6 0 7 . 5 9 . 0 1 0 ; . . . . . . ; . . . o 11 2 0 1 3 5 K ( M P a ~ / m )

K ( M p a ~ / m )F I G . 4 . K - v r e s u l t s f r o m f i x e d - g r i p te s t s s h o w i n gm and n - s haped cu rves rep re s en t ing t r ans ien tand in t e rmi t ten t c rack g rowth .

F I G . 5 . K - v p l o t f r o m a f i x e d - l o a d te s t( inc rea s ing K wi th c rack g rowth ) .The loopoccur red due to a com bina t ion o f a t r ans ien tc r a c k a c c e l e r a t i o n a n d t h e f a i l u r e o f t h el o a d i n g s y s t e m t o m a i n t a i n c o n s t a n t l o a dcontrol during the t rans ient .

001

00014 5

i I i i i' ~ 7 1 .50 55 6.0 - 65 5

K ( M P a ~ m )

FIG . 6 . R es u l t s o f 5 run -a r re s t even t s in a s ing le s pec imen s how ing non-un ique K-v .The duc t i l e d imp le f rac tu re appea rance , cha rac te r i s t ic o f in -a i r f r a c tu re s u r face topography o f a luminum,changes d ram a t i ca l ly when in con tac t w i th mercu ry , a s ha s been repo r t ed e l s ewhe re (9 ). Some o f the f rac tog raph icfea tu re s a re read i ly recogn izab le a s bo th c l eavag e and in t e rg ranu la r a s we l l a s ve ry s m a l l duc t i l e d imp le s a f t e rf rac tu re in m ercu ry . S om e add i t iona l f e a tu re s a re wor th no t ing . F igu re 7 and a p a r t o f F igu re 8 s how the f ractu rea s a s e t o f f i a t , rough ly pa ra l l e l , bu t non-cop lana r s t eps . R unn ing d iagon a l ly ac ros s F igu re 8 f rom the top r igh t tothe bo t tom le f t i s a f e a tu re tha t ma rks an a r re s t ed c rack f ron t . The c rack had been p ropaga t ing in con tac t w i thmercu ry f rom f ig h t to l e f t, a r re s ted , t he mercu ry remo ved , and the s am ple was then b roken in a i r , p roduc ing , in

F igu re 8 , c l e a vage - l ike appea ran ce on the f igh t and duc t i l e d imple on the l e f t . H igh e r magn i f i c a t ion in F igu re 9s h o w s a f i v e r p a t t e r n s u g g e s t i v e o f c l e a v a g e a s w e l l a s e v i d e n c e o f l o c a l i z e d d u c t i l e r u p tu r e . ( T h e c r a c kp r o p a g a t i o n d i r e c t i o n i s f i g h t - t o - l e f t i n a l l c a s e s ) . F r e q u e n t l y , w i t h m e r c u r y , d e l a m i n a t i o n o f t h e s a m p l e a l sooccur red b y one o r m ore c rac ks p ropag a t ing , on l.y in t e rg ranu la r ly , on p lanes rough ly pa ra l l e l to the ro l l ing p laneas s hown in F igu re 10 . Indeed , in s om e ca s e s g rmns w e re comp le te ly enc i rc l ed by the s e in t e rg ranu la r c racks andbecame de tached f rom the s u r face . F igu re 10 a l s o c l ea r ly s hows the non-cop lana r s t eps on the ma in f rac tu res u r face (a r rowed r igh t edge ) . Som e exam ple s o f de tached g ra ins a re s hown in F igu re 11 wh ich a l s o p rov ide s afa i r ly c l ea r ind ica t ion o f the typ ica l g ra in s hape and s i z e. A com mon ly obs e rv ed fea tu re on the s u r face was s hea rwa l l s wh ich a re the nea r ly ve r t i c a l r i s e s in the s t epped s u r face and wh ich d i s p lay ev idence o f duc t i l e rup tu re


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Figur e 12) . A s d i scusse d be low , w e be l ieve the f r ac ture of these w a l ls l ags behind the embr i t t l ed c r ack andconsequently these walls act as ligaments.

F I G . 7 . T y p i c a l f r a c t u r e s u r f a c e o f a l u m i n u membr i t t led by m er cur y show ing n on- coplanar s teps andcleavage-like features.

F I G . 8 . A v i e w o f a r e g i o n o f t r a n s i t i o n f r o mme rcury embf i t t led f racture f ight) to air f ractureleft).

F I G . 9 . Cleav age f ive r pa t te r n and ev idenc e oflocalized ductile dimples.

FIG . 10. A transverse section perpendicular to crac k propagation d irection) sh ow ing intergranular delaminationand non -coplan ar s teps on the main f racture sur face r ight edge) .


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3 4 4 E M B R I T T L E M E N T O F A L B Y H g V o l . 2 3 N o . 3

F I G . 1 3 . S c h e m a t i c o f t h e r e la t i o n b e t w e e n t h e g r a i nm o r p h o l o g y a n d t h e c r e a t i o n o f f ia t , n o n - c o p l a n a r s t ep ss e p a r a t e d b y s h e a r w a l l s a n d i n t e r g r a n u l a r f r a c t u r eperpendicular to the m ain crack plane.

W h i le w e expec t to publ ish a comple teanalysis o f the l igamen t ef fect in the nea r future,as a quali ta t ive explanation, we suggest that thef or mat ion and sub sequent r up tur e o f l igaments ,shear-walls, etc, as the crac k extend s is a processthat produces a continually varying conf igurationand the r e f or e , a cont inua l ly changing sh ie ld ingcont r ibu t ion . The r esu l t i s tha t the sh ie ld ingcontribution var ies s tocha st ical ly and, even if theK-v relation w ere unique where K here refers tothe loca l K ) , the decoupl ing f r om the appl ied Kp r o d u c e s a n a p p a r e n t n o n - u n i q u e r e l a t i o nbetween th e applied K and the crack veloci ty.F r a c t o g r a p h i c a n a l y s i s s h o w s t h a tL M E c r a c k s p r e f e r t h e p a t h s t h r o u g h g r a i nboun dar ies , i f the specim en or ientation favors i t.This is con sis tent with mo st f requent repor ts . I f

the orientat ion of the specime n does no t favo r anin te r granula r pa th , a s f or exam ple in th is w or k ,c leavage appear s to occur . Ther e r emains someques t ion as to the c r ys ta l logr aphic char ac te r o ft h i s f r a c t u r e , h o w e v e r , a s c l e a v a g e i n F C Cmetals is quite unu sual.Conc lus ions

Cr ack pr opag a t ion te s ts o f 7075- T651 a luminum em br i t fl ed by l iqu id mer cur y d i sp lay a spec t rum oft ransien t and in te r mi t ten t c r a ck gr ow th beha vior sugges t ing the the c r ack ve lo c i ty i s, a t bes t, w eakly dep endentupo n the applied s tress intensi ty. Wh ile concu rrent crack tip oxidat ion has been pro pose d as a contributing factor ,f r ac togr aphic f ea tur es ind ica te tha t l igamen t f or mat ion and r up tur e i s a l so a po ten t ia l explana t ion f o r the non-unique K - v r e la tion f or th i s a l loy . SEM f r ac togr aphs show tha t mer cu r y- LM E f r ac tur e of thi s a l loy occur r ed byintergranular cracking, localized shearing and transgranular cleavage.A eknow ledgment

Th e authors w ish to thank Dr . J. H. Larsen, Mr . Br ian M iller , M s. Chr is Davit t and M s. Joyce Davis foras si stance . T his w or k w as suppor ted by the D epar tment of Ener gy under cont r ac t D E- FG 06- 87ER 45287 .References

1. T .P . S la vin and N.S. Stolo f f , Mat. Sci. Eng . , 68, 55, 1984) .2 . N . S . S to lof f , A tom is t ic s of F r ac tur e , P . 921 , N A T O Conf . P r oc ., R . M. La tan is ion , J . R . P ickens,eds . , 1982) .3. S .P . Ly nc h, Scr ipta Met. , 13, 1051, 1979) .4 . S .P . Lync h , J . Mat . Sc i . , 20 , 3329 , 1985).5 . C . F . O ld , Meta l Sc ience , A ug - Sep , 433 , 1980).6 . S .P . Lynch , A c ta Met . , 29 , 325 , 1981).7 . M .H . K amdar . F r ac tur e in L iquid Meta l Envi r onm ents , P . 3815 , I CF6, P . R . Ran e t a l eds. , 1984).8. M .J . Kelle y, N. S . S tolo f f , Me t. Trans . , 6A , 159, 1975) .9 . D .A . W hee le r , R . G . H oag land , Sc r ip ta Met ., 20 , 1433 , 1986).10. M. F . Kan ninen , Int . J . Fract . , 9, 83, 1973) .11. J . A. Kap p, D. Daq uette an d M. H. Kamd ar , J. Eng . Mat. Tech. , 108, 27, 1986) .12 . M. O . Spe ide l , Th e The or y of S t r es s Cor r os ion Cr acking in A l loys , P . 289 , N A T O Conf . P r oc . ,J . C. Scu lly, ed. , 1971) .13. R. G. Ho agla nd, A. R. Rosenf ield , and G. T. Hah n, Metall . Trans . , 3, 123, 1972) .14 . G . T . H ahn, R . G . H oagland , J . Ler e im, A . J . Ma r kw or th , and A . R . Rosenf ie ld , A ST M - STP 711 , P . 289 ,1980).15. A. R. Ro senf ie ld and B. S . M ajum dar , M etall . Trans ., 18, 1053, 1987) .16. R. H. Jones , J . Metals , Dec. , 32, 1987) .

Transient and Intermittent Crack Growth During Embrittlement of 7075-T651 Aluminum by Mercury

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